Methods of and apparatus for fracturing



y 1954 c. M. HUDGINS, JR., ETAL 3,139,930

METHODS OF AND APPARATUS FOR FRACTURING 2 SheetsSheet 1 Filed Jan. 8, 1962 INVENTORS E3 M. HUDG/NS,

CHA RL ROBERT L. MCGLASSON WA N BY 0 an mouse ATTORNEY TUE-1 y 1964 c. HUDGINS, JR., ETAL 3,139,930

METHODS OF AND APPARATUS FOR FRACTURING Filed Jan. 8. 1962 2 Sheets-Sheet 2 INVENTORS OIARLES M. HUDG/NS, R RMRT L. MCGLASSON WALTON 0. GREAWOUSE ATTORNEY United States Patent 3,139,930 METHODS OF AND APPARATUS FOR FRACTURING Charles M. Hudgins, Jr., and Robert L. McGlasson, Ponca City, Okla., and Walton D. Greathouse, Houston, Tex.,

assignors to Continental Oil Company, Ponca City,

Okla., a corporation of Delaware Filed Jan. 8, 1962, Ser. No. 164,668 15 Claims. (Cl. 166-36) The present invention relates to methods of and apparatus for fracturing, and more particularly, but not by way of limitation, relates to an improved method and apparatus for fracturing a producing formation around a well bore by using gas under impact pressure loading conditions.

As is well-known in the petroleum industry, numerous techniques are presently utilized for fracturing the producing formations around a well bore in order to increase the productivity of the oil or gas well. All of the presently known techniques involve the use of a liquid-phase fluid and, essentially, involve the concept of pumping the liquid into the formation at a very high pressure over a relatively long period of several minutes. The pressure of the liquid ruptures or cracks the producing strata simply by slowly applied brute force. Propping agents, such as sand, and also various types of fluid loss control agents may be utilized to increase the efficiency of the particular fracturing technique in a particular type of formation. While these various techniques employing liquid-phase fluids have proven successful to a certain extent, they do not possess the advantages of the methods and apparatus described hereinafter.

It is generally well-known in the physical sciences that impact loading is nearly always more destructive to a material than slow loading. A collection of data from various studies indicates that the rate and depth of crack propagation through a particular material as a result of a pressure load applied by a fluid medium in contact with the surface of the material is primarily dependent upon the following four factors: (1) the magnitude of the pressure load; (2) the length of time required to reach peak load conditions and the duration of the peak load conditions; (3) the physical properties of the medium through which the pressure load is applied to the material to be fractured; and (4) the physical properties of the material which is to be fractured.

Therefore, it will be noted, the depth of penetration of cracks caused by the fracturing process will be dependent not only upon the total energy applied by the impact load but also upon the ability of the formation rock to absorb the energy of the impact. By way of example, identical blows to a block of lead and a block of ice may only dent the block of lead but split the block of ice in half. Therefore, when it is desired to fracture the usually brittle rock of a producing formation, more extensive fractures may be expected from application of an impact pressure load to the formation rather than from application of a pressure load which builds to the peak pressure slowly. There is also considerable evidence that brittle rock will fracture more readily when enveloped by gases than the liquids. However, use of gases instead of liquids in previously known techniques would be ineffective because the gas would diffuse into the formation before a sufficiently high pressure could be attained.

Attempts have heretofore been made in the art to apply an impact load to a subsurface formation around a well bore through the medium of a liquid-phase fluid. However, due to the incompressibility of liquids, it is 3,139,930 Patented July 7, 1964 ice extremely diflicult, from a mechanical standpoint, to generate the energy required to fracture the formation. This problem may be due to the fact that although a high magnitude impact pressure load can be very quickly produced, since only a minimum movement of a compressing piston is required to generate the high pressure, the pressure is just as quickly dissipated by the slightest leak of the liquid into a crack created in the formation.

Therefore, it is contemplated by the present invention to provide an improved method and apparatus for fracturing subsurface formations around a well bore by suddenly increasing the pressure of a fracturing fluid within a well bore at a suflicient rate to limit ditfusion of the fluid into the formation and to a high pressure to apply an impact pressure load sufiicient to fracture the formation.

It is also contemplated by the present invention to provide apparatus for suddenly increasing the pressure of a fluid in a well bore whereby the surrounding formation is fractured by an impact pressure load. The apparatus may comprise a ballistic piston slidably received in the well bore, means for lowering the piston into the bore to a point above the formation and then for holding the piston in a fixed position. Pressure is then built up in the fluids within the well bore above the piston which is greater than the pressure of the gas within the bore below the piston. Releasable coupling means are provided for releasing the piston from the means for lowering the piston so that the pressure above the piston, with an assist from gravity, will force the ballistic piston rapidly downwardly to compress rapidly the gas within the bore below the piston and thereby apply an impact pressure load to fracture the formation which is in fluid communication with the well bore.

It is further contemplated by the present invention to provide a method and an apparatus of the type described by which liquid standing in the lower portion of a casing string can be displaced and the casing below the piston entirely filled with gas. It is also contemplated by the present invention to provide a novel means for generating a greater pressure in the casing above the piston than below the piston by combustion of a fuel and, if desired, for using a reaction propulsion force to assist in propelling the ballistic piston downwardly through the casing string to increase rapidly the pressure of the gas below the piston and thereby apply an impact gas pressure load to the formation.

Therefore, it is an important object of the present invention to provide an improved method for fracturing a formation from a well bore.

Another object of the present invention is to provide a method for fracturing subsurface formations from a well bore by application of an impact pressure load.

Another object of the present invention is to provide a method for fracturing a formation from a well bore in which liquids are standing to a substantial depth.

Another object of the present invention is to provide a novel method of the type described in which the pressure of gas in the well bore above the piston is rapidly generated by combustion of a self-sustaining fuel, such as a slow burning explosive or rocket fuel.

Another object of the present invention is to provide a novel device for creating an impact gas pressure load to fracture formations from a well.

Another object of the present invention is to provide a device of the type described having a novel valving arrangement by which liquid standing in the casing can be displaced or swabbed from the casing so that the casing below the piston can be at least partially filled with gas.

Still another object of the present invention is to provide a device of the type described which utilizes the pressure generated by combustion of a fuel and also, if desired, the reaction force from combustion of the fuel, for propelling the ballistic piston downwardly to compress the gas in the well bore below the piston.

Many additional objects and advantages of the present invention will be evident to those skilled in the art from the following detailed description and drawings.

In the drawings:

FIG. 1 is a vertical cross sectional view of a device constructed in accordance with the present invention.

FIG. 2 is a vertical cross sectional view of a modification of the device of FIG. 1, the modification of FIG. 2 having the same reference numerals as the device of FIG. 1 where appropriate.

FIGS. 3, 4 and 5 are schematic vertical sections of the lower end of a well bore and serve to illustrate the methods of the present invention as practiced by the novel de vice of FIG. 1.

FIG. 6 is a schematic vertical section of the lower end of a well bore and serves to illustrate the method of the present invention as practiced by the modification of FIG. 2.

Referring now to the drawings, and in particular to FIG. 1, a string of casing 10 may be disposed in a well bore 12 which is drilled through subsurface geological formations indicated generally by the reference numeral 14. The string of casing 10 may be anchored in the well bore 12 by any conventional means, such as cement, and the casing string will normally be of a uniform inside diameter throughout its length.

A device constructed in accordance with the present invention is indicated generally by the reference numeral 16 and comprises what may be termed a ballistic piston, indicated generally by the reference numeral 18, shown slidably received in the casing 10, and means for lowering the piston 18 into the casing string 10, such as a tubing string 20. The ballistic piston 18 preferably has a body 19 of substantial mass which will produce a high inertial force when accelerated to a high speed. The piston 18 receives the end of the tubing string 20 in a bore 21 and is connected to the tubing string 20 by a releasable coupling means indicated generally by the reference numeral 22. The tubing string 20 may be of the conventional type consisting of a number of pipe joints threaded together and in addition to serving as a means for lowering the ballistic piston 18 into the casing string 10, the tubing string serves as a means for introducing gas into the casing 10 below the piston 18, as hereafter described.

The releasable coupling 22 may comprise frangible shear pins 23 and 24 which may be threaded into transverse bores 25 and 26 through the body 19 of the piston 18 by conventional Allen head screws. The pins 23 and 24 extend into receiving apertures 27 and 28 in the lower joint of the tubing string 20. A pair of O-ring seals 30 and 32 provide a fluid tight seal at the releasable coupling 22 between the tubing and piston so that pressures can be applied as hereafter described in detail. The shoulder 33 in the bore 21 prevents accidental or premature shearing of the pins 23 and 24 due to jamming of the piston as it is lowered into the casing 10.

The ballistic piston 18 may be provided with a plurality of annular sealing rings 36 which are seated in annular grooves 38 in the piston body 19. The annular sealing rings 36 may be of any conventional construction for providing a fluid tight seal between the ballistic piston 18 and the interior walls of the casing string 10 to permit establishment of a pressure differential across the piston 18 as hereafter described.

The piston body 19 has a longitudinally extending bore 40 which, together with bore 21, forms a fluid passageway extending completely through the piston 18 for fluid communication between the interior of the tubing string 20 and the interior of the casing string 10 below the piston 18. The longitudinally extending bore 40 preferably has a restricted portion 42 so as to form an upwardly facing valve seat 46. An upper free spherical valve body 44 is provided to seat on the upwardly facing valve seat 46. The upper valve body 44 is of a size to be passed downwardly through the tubing string 20 and through the upper portion of the bore 40 and seat on the valve seat 46 after the ballistic piston 18 has been positioned in the casing string 10 and gas pumped down through the tubing and bore 40 and lower valve assembly 47 as hereafter described in detail.

A lower valve assembly, indicated generally by the reference numeral 47, has a valve cage 48 which is retained in a counterbore 49 of the longitudinally extending bore 40 by an insert 50 which is threaded into the counterbore 49. The insert 50 has a bore 51 for passing fluid therethrough into the bore 40. A lower valve body 52 has a stem 53 which is slidably retained in a support'spider 54 of the cage 48. A nut 530 may be provided on the upper end of the valve stem 53 to prevent the valve stem from leaving the spider support. The valve body 52 is adapted, upon upward movement thereof, to seat on a downwardly facing valve seat 55 formed on the valve cage 48, in order to block the upward passage of fluid through the bore 40 and therefore through the piston 18, as hereafter described. A coil spring 56 is provided around the stem 53 between the spider 54 and the head of the valve body 52 to normally bias the valve body 52 downwardly from the seat 55 into the open 'position. The spring is sufficiently strong to maintain the valve open as the piston 18 is slowly lowered through fluid standing in the casing string and thereby permit the fluid to pass upwardly through the bore 40 into the tubing string 20, but is sufficiently Weak as to permit the valve to close when the piston 18 is moved downwardly at a high rate of speed in order to compressthe gas in the casing string 10 below the piston 18, as hereafter described in greater detail.

A pair of ducts or passageways indicated generally by the reference numerals 59 and 60 provide fluid communication between the interior of the tubing 20 and the casing annulus above the piston 18. The passageways 59 and 60 are comprised of radially extending bores 61 and 62 in the piston body 19 which intersect longitudinally extending bores 64 and 66, respectively, which in turn extend to the upper face 68 of the piston body 19. Valve seats 70 and 72 are formed in the radially extending bores 61 and 62, respectively, by threaded counterbores 74 and 76. Ball-type valve bodies 78 and 80 are positioned in the counterbores 74 and 76 and are biased against the valve seats 70 and 72 by springs 82 and 84. The springs 82 and 84 may be retained in the counterbores 74 and 76 by Allen head plugs 86 and 88 which are threaded into the respective counterbores. The springs 82 and 84 should have suflicient strength to maintain the valve bodies 78 and 80 seated and thereby block fluid passage from the interior of the bore 40 to the casing string 10 above the piston 18 until a substantial pres- (siure is built up in the bore 40, as hereafter described in etail.

In the ballistic piston (see FIG. 2) the piston body 19 is further modified by means for providing a downward thrust such as a pair of rocket engines 102 having upwardly directed nozzles 103. The rocket engines may be filled with a conventional rocket fuel 104 or other suitable slow burning explosive. The combustible rocket fuel 104 may be ignited at a desired time by a conventional ignition means, such as an electrical ignition system represented by electrical leads 106 extending to the wellhead or by a self-contained timing mechanism (not shown). Upon ignition, the fuel 104 burns rapidly to generate gases of combustion which jet through nozzles 103 to increase the pressure of the fluid in the casing annulus above the piston 100 and also provide a reactive propulsion force to assist in accelerating the piston 100 downwardly through the casing 10.

FIGS. 3, 4 and 5 illustrate the manner in which the device 16 may be operated to apply a gas impact pressure load to a producing formation 112 in accordance with the method of the present invention. The casing str1ng 10, which is disposed in the well bore 12, may have perforations 110 or other apertures adjacent to theproducing formation 112. The perforations 110 will provide fluid communications between the interior of the casing string and natural cracks 114 in the formation 112 where such exist, or in the absence of such cracks with a face of the formation, or at a zone of prior perforation. The casing 10 and the well bore 12 may terminate at a cement plug 116, or any other suitable bottom hole finishing method. Alternatively, the casing 10 may extend below the producing formation which is to be fractured, in which case a suitable conventional packer (not shown) may be inserted in the casing string 10 below the perforations 110 to block the passage of fluid down the casing 10 and thereby permit fracturing of the formation 112. The producing formation 112 will normally have a natural pressure which will, in most cases, cause a quantity of liquid-phase fluid to rise in the casing 10 to a substantial depth, and in some instances to the wellhead.

The ballistic piston 18 is connected to the lower joint of a string of tubing 20 by the releasable coupling 22. The piston 18 is then slowly lowered by the tubing string 20 into the casing string 10 to a point just above the perforations 110, as shown in FIG. 3. The piston 18 must be lowered somewhat slowly so that the spring 56 will maintain the valve body 52 off the seat 55 and permit the fluid, either gas or liquid, standing in the casing string to pass upwardly through the bore 40 and into the tubing 20, or in some instances through valves 78 and 80 into the casing above the piston as the piston 18 is lowered. The free valve body 44 is retained at the wellhead when the ballistic portion 18 is first lowered to the position shown in FIG. 3 and is not inserted in the tubing until the casing below the piston is filled with gas as hereafter described.

After the piston 18 is lowered into the casing through the liquid, which will normally be standing in the casing 10, gas may be pumped down through the tubing string 20, through the bore 40, past the valve assembly 47 and into the casing 10 below the piston 18. As .the gas is pumped down through the tubing, any liquid which has risen in the tubing 20 will either be forced back into the formation or will force the valves 78 and 80 open and will then pass through the passageways 59 and 60 into the casing annulus above the piston. The pressure of the gas is preferably increased until it exceeds the natural pressure of the producing formation 112, and is maintained greater than the formation pressure to insure that additional liquids do not enter the casing 10 below the piston. The piston 18 is then raised by the tubing string to swab the liquid in the casing annulus above the piston from the lower end of the casing. The piston 18 readily operates as a swab because of the annular rings 36 and the check valves 78 and 80.

While swabbing the casing in this manner, it may be necessary to raise the piston 18 to a height h (see FIG. 4) above the plug 116 as shown in FIG. 3, which will require one or more joints of the tubing string 20 to be disconnected at the wellhead. In this case it would be difficult to maintain the pressure in the casing 10 below the piston 18 at a pressure greater than the formation pressure except for the operation of the check valve assembly 47 in the lower end of the bore 40. By increasing the pressure of the gas below the piston 18 to a substantial value and then suddenly releasing the pressure in the tubing 20, the pressure below the piston 18 will seat the valve body 52 and permit the upper joints of the tubing string to be disconnected from the tubing string as the piston is raised to the desired height. In order to increase the pressure below the piston 18 to this high pressure, it may be necessary to close the casing 10 at the wellhead so that a large pressure differential will not open the valves 78 and 80. Of course, the pressure below the piston 18 can be increased to any less than fracturing pressure which may be desirable after the piston is raised to the height h above the cement plug 116 or fluid packer (not shown), as the case may be.

After the piston 18 has been raised to the desired height h and the pressure of the gas in the casing below the piston is as desired, the spherical, free valve body 44 is introduced to the tubing string 20 at the wellhead and either falls by gravity or is pumped downwardly through the tubing string 20 and seats on the upwardly facing valve seat 46 to block further downward passage of gas through the bore 40 in the piston body 19. As the pressure in the tubing string 20 is increased, the valve bodies 78 and 80 are opened against the bias of the springs 82 and 84 and the gas is pumped through the passageways 59 and 60 into the casing annulus above the piston 18. The casing string 10 above the piston may be closed at the wellhead at the time the valve 44 is dropped into the tubing string 20 provided that a sufficient volume of gas is present in the casing above the piston 18 at that time. However, if the casing is standing full of liquid, a certain volume of the liquid should be forced from the casing annulus above the piston before the casing is closed at the wellhead. This will insure that a sufficient volume of gas is accumulated above the piston 18 to expand and supply the necessary energy impulse to accelerate the ballistic piston 18 downwardly as hereafter described. As will be evident from formulas hereafter set forth, the volume of compressible fluid, that is gas, should be a maximum for most efficient operation.

After a suflicient volume of gas is assured in the casing 10 above the piston 18, the casing 10 may be closed at the wellhead and the pressure of the fluids above the ballistic piston 18 increased preferably to a value several times that of the pressure of the gas in the casing 10 below the ballistic piston 18. When the pressure differential across the piston 18 has increased sufliciently to shear the frangible shear pins 23 and 24 of the releasable coupling 22, the ballistic piston 18 will be separated or released from the tubing string 20. Then the substantially greater pressure of the fluids in the casing above the piston, in conjunction with the force of gravity, will fire or accelerate the ballistic piston 18 downwardly through the casing string 10 to rapidly compress the volume of gas in the casing 10 below the ballistic piston 18 to a much higher pressure. The great momentum of the high mass ballistic piston 18 will generate a much higher pressure than originally existed in the casing annulus above the piston. As the gas pressure in the casing 10 is rapidly increased, an impact pressure load will be applied to the producing formation 112 by the gas passing from the interior of the casing 10 through the perforations and into the formation.

It will be noted that during the first portion of the downward movement of the ballistic piston 18, as illustrated in FIG. 5, the high pressure above the piston 18, which upon separation of the piston 18 from the tubing 20 is introduced to the bore 40, will keep the valve body 44 firmly on the seat 46. Of course, the annular sealing rings 36 will prevent passage of fluid through the annulus between the casing 10 and the piston 18. Then as the pressure below the piston 18 becomes greater than that above the piston, the check valve assembly 47 will close so that a much greater pressure can be built up in the casing below the piston 18 than exists at that time in the casing above the piston.

It will be obvious to those skilled in the art that in the event a column of liquid is not standing in the lower end of the casing, it will then be unnecessary to lower the piston 18 to a point just above the perforations 110 and to then swab the fluid from the casing by lifting the piston and injecting gas under pressure into the casing 10 as the piston is raised. Instead, the piston may be lowered only to its final position at a distance h above the point where the casing is sealed, and the casing below the piston 18 then pressured as desired before the pressure above the piston 18 is increased to provide the propulsion force for the ballistic piston. This will also be the case where the producing formations are such that a column of fluid standing in the casing can easily be forced or injected back into the producing formation to thereby fill the casing below the piston with a gas. Also, it will be evident that in the event it is unnecessary to swab the liquid from the lower portion of the casing as described, a ballistic piston could be lowered into the casing by means of a wire line, or any other suitable means for suspending the piston at the desired height h above the point where the casing 10 is closed, and then the pressure above the piston and within the casing annulus increased to the desired pressure to break a suitable releasable coupling between the wire line or other lowering means and the piston so that the pressure differential would accelerate the piston downwardly as described.

Additionally, an increase in fracture size may be achieved by first displacing the fluid in the fiuid-filled pores of a portion of the formation with gas. This can be done by continuing the pressuring up phase for any period of time required to displace the desired volume. This effect would normally occur in low pressure, high permeability formations, although it can be caused to occur to a predetermined extent in any formation.

The operation of the ballistic piston 100 is substantially identical to the operation of the device 16 up to the point that the pressure in the casing annulus above the piston 100 is to be increased. The ballistic piston is inserted in any suitable manner as described in regard to the device 16 in order to establish a column of gas in the casing string below the piston at the desired pressure. Then the fuel 104 in the rocket engines 102 is ignited. As the gases produced by combustion of the rocket fuel 104 are jetted upwardly, the pressure above the ballistic piston 18 will quickly increase to provide a sufficient pressure differential across the piston 100 to shear the pins 23 and 24 and release the ballistic piston 100 from the tubing string 20. It may be desired to provide a sufficiently long burning supply of fuel 104 as to continue burning after the piston 100 has separated from the tubing string 20 to provide reaction propulsion or thrust for assisting in propelling the piston 100 downwardly through the casing string 10.

If desired, the shear pins releasably connecting the piston 100 to the tubing 20 could be so arranged and fabricated as to be thermally released as a result of the heat of combustion of the fuel 104. It will also be evident that if the reaction or rocket thrust is not desired, the fuel 104 could be located within the casing, or in communication with the casing at the wellhead, and upon combustion would suddenly produce the desired high propulsion pressure in the casing annulus above the piston 100. However, locating the combustible fuel within the ballistic piston, even when the reaction propulsion feature is not desired, eliminates the friction losses which would be created as a result of the combustion gases flowing down the casing string to act on the piston as would be necessary in the event the fuel were burned at the wellhead. However, by utilizing the combined accelerating forces of the greater pressure in the casing annulus above the ballistic piston 100, the thrust of the rocket engines 102, and the assistance of gravity, it will be evident that a pressure below the piston can be generated in a very short time which is much greater than the maximum existing pressure in the casing above the ballistic piston 100.

From the above detailed description of the method and apparatus of the present invention, it will be evident to those skilled in the art that a large number of factors must be considered from an engineering standpoint in order to efficiently practice the invention. For example, there are many factors which will directly affect the extent to which a particular formation will be fractured such as: the weight of the ballistic piston 18; the pressure 9 wherein:

g=acceleration due to gravity (ft/sec?) M=weight of the piston (1b.) u=velocity of the piston (ft/sec.) 0=time (sec.) P =pressure above the piston (lb/ft?) P =pressure below the piston (lb/ft?) A=cross sectional area of the piston (sq. ft.) F :frictional force resisting movement of the piston (1b.)

Of course, a reaction impulse of force should be added in the event the rocket motors 102 are used. From Equation 1, by assuming the weight of the piston is constant and thereby disregarding the weight of any liquid-phase fluid above the piston which may be adding its weight to that of the piston, and by using the relationship u dh/dti, wherein it equals the distance in feet measured upwards from the point the casing string is closed, the following equation results:

However, since Adh=dV, when V and V equal the volume of the casing above and below the piston, respectively, Equation 2 may be written as:

Further, if the gas under the piston is considered to be a perfect gas, and the gas is considered to undergo an adiabatic compression, the following Equation 4 can be used to assist in determining the height h at which the piston should be positioned above the point where the casing 10 is sealed and the pressures which should be developed in the casing above and below the piston:

In Equation 4, the subscripts 1" and 2 denote the conditions before and after compression of the gas below the piston, which would be before and after release of the piston 18 from the tubing string 20. In Equation 4, the quantity k is a constant for any particular gas, and for most gases is about 1.4, as can be attained from many available thermodynamic tables.

In order to properly design the ballistic piston 18 or 100, insofar as its weight or mass is concerned, and in order to determine where to position the piston in the casing and what pressures must be established in the casing above and below the piston in order to obtain the desired results, it is necessary to know the breakdown pressure of the rock of the formation which is to be fractured. In general, the breakdown pressure of the rock can be closely approximated from the depth of the formation. The breakdown pressure gradients are approximately 1.5 to 2.0 p.s.i. per foot of depth from 0 to 1000 feet; approximately 1.0 p.s.i. per additional foot of depth from 1000 to 3000 feet; and, something less than 1.0 p.s.i. per additional foot of depth below 3000 feet. In order to insure fracturing of the formation, the design maximum pressure should exceed the estimated breakdown pressure by a considerable margin. By exceeding the estimated breakdown pressure, some compensation can be made for the many factors which cannot be calculated with any degree of certainty. For example, the volume of gas and pressure loss which will result from leakage or diffustion of the gas into the formation will vary with each well. The friction between the annular sealing rings 36 around the piston 18 and the walls of casing string will vary considerably with the condition of the interior walls of the particular casing string.

However, some relatively rough working estimates can be obtained by trial and error calculations from the above equations and information. The pressure required to fracture the formation, which will be P of Equation 4, can be estimated from the depth of the formation by using the pressure gradients set out above. The pressure P below the piston before release of the piston should be kept as low as possible and yet prevent entrance of liquid fluid into the casing and into the well bore at the producing formation, this pressure normally being slightly greater than the natural pressure of the formation. By selecting these two pressures, the ratio between the volumes of the gas below the piston before and after the piston is released can be determined. From this ratio, the distance the piston must be moved in order to establish the higher pressure P can be estimated. Then by trial and error calculations, it can be determined what pressure and volume of gas will be required above the piston in order to propel a piston of a certain mass downwardly to the point in the casing 10 necessary to produce the second volume V and corresponding pressure for fracturing the formation.

From the above detailed description, it will be evident that a novel method for fracturing a formation in a well bore has been described which can be expected to have considerably greater efficiency in a more brittle formation. The high gas pressure can be generated very quickly to apply an impact pressure load to the formation at a sufficient rate that the gas cannot diffuse into the formation to such an extent as to appreciably reduce the energy applied by the impact pressure load. Yet the peak magnitude of the impact load will persist for a considerably longer time than such a load applied by a liquid fluid under similar conditions because of the compressibility of the gas. The compressibility of the gas of course reduces the instantaneousness with which the impact load can be applied. On the other hand, the gas expands into the cracks created in the formation by the impact load to provide a follow-through action and project the cracks deeper into the producing formation. A novel apparatus has also been described by which the novel method can be practiced under most well bore conditions, and in particular, in the usual circumstance where a liquid is standing in the casing string to a substantial depth.

Although particular embodiments of our invention have been described in detail, it is to be understood that various changes and substitutions can be made therein without departing from the spirit and scope of our invention as defined by the appended claims.

We claim:

1. A method for fracturing a subsurface formation from a well bore comprising the steps of placing a piston in the well bore at a position substantially above the formation;

placing in the well bore below the piston a gas at a first pressure;

increasing the pressure of fluids in the well bore above the piston to a substantially greater pressure than the first pressure while supporting the piston at said position; and,

releasing the piston;

whereby the pressure of the fluids above the piston force the piston rapidly downwardly to produce an impact gas pressure load for fracturing the formation.

2. A method for fracturing a subsurface formation from a well bore having a string of easing therein, the casing having openings therein communicating with the well bore at points adjacent the formation to be fractured, the method comprising the steps of:

positioning a piston in the casing at a distance above the openings in the casing; passing gas at a first pressure through the casing to a point below the piston; placing gas in the casing above the piston at a pressure substantially greater than the first pressure while supporting the piston to prevent it from moving relative to the casing; and, releasing the piston; whereby the piston moves rapidly downwardly to produce an impact pressure load on the formation. 3. A method for fracturing a subsurface formation from a well bore having a string of casing therein, the casing having openings therein communicating with the well bore at points adjacent the formation to be fractured, the method comprising the steps of:

positioning a piston in the casing at a point adjacent said openings therein; pumping gas into the casing below the piston while raising the piston to an upward position as the casing is filled with gas, the pressure of the gas below the piston being greater than the formation pressure around the well bore; placing gas in the casing above the piston at a pressure substantially greater than the pressure below the piston while retaining said piston at said upward position; and, releasing the piston. 4. A method for fracturing a subsurface formation comprising the steps of:

lowering a piston into the well bore to a point adjacent said formation; pumping gas into the well bore below the piston while raising the piston to an upward position as the bore is filled with gas, the pressure of the gas below the piston being greater than the formation pressure around the well bore; pumping gas into the well bore above the piston until the pressure above the piston is substantially greater than the pressure below the piston while retaining said piston at said upward position; and, releasing the piston; whereby the piston moves rapidly downwardly to produce an impact pressure load on the formation. 5. A method for fracturing a subsurface formation from a well bore comprising the steps of:

positioning a piston in the well bore at a point above said formation; placing in the well bore below the piston a gas at a first pressure; burning fuel in the well bore to produce a pressure therein above the piston substantially greater than the first pressure while maintaining the position of said piston at said point during at least the initial portion of time that said fuel is burning; and, releasing the piston; whereby the piston moves rapidly downwardly to produce an impact pressure load on the formation. 6. A method for fracturing a subsurface formation from a well bore comprising the steps of:

lowering a piston into the well bore to a point adjacent said formation; pumping gas into the well bore below the piston while raising the piston to an upward position as the bore is filled with gas, the pressure of the gas below the piston being greater than the formation pressure around the well bore; burning fuel in the well bore to produce a pressure above the piston substantially greater than the pressure below the piston while maintaining the position of said 1 1 piston at said point during at least the initial portion of time that said fuel is burning; and,

releasing the piston;

whereby the piston moves rapidly downwardly to produce an impact pressure load on the formation.

7. A method for fracturing a subsurface formation from a well bore comprising the steps of:

lowering a piston having an upwardly directed fuel containing reaction engine therein into the well bore;

filling the well bore below the piston with a gas at a first pressure;

igniting fuel in the upwardly directed reaction engine to generate a pressure in the well bore above the piston substantially greater than the first pressure while maintaining the position of said piston at said point during at least the initial portion of time that said fuel is burning; and,

releasing the piston at or before the time at which ignited fuel ceases to burn;

whereby the piston moves rapidly downwardly by the combined forces of the pressure differential across the piston, the thrust of the reaction engine and the force of gravity to rapidly compress the gas in the well bore below the piston and produce an impact pressure load to fracture the formation around the well bore.

8. Apparatus for fracturing a subsurface formation from a well bore in which a casing string is disposed, said string having an opening therein for communication between the interior of the casing string and the formation to be fractured, the apparatus comprising:

a tubing string extending downwardly through the casa piston received within the casing,

said piston including a vertically extending fluid passageway formed therethrough and further including a duct therein, said duct communicating with said passageway and with the exterior of said tubing string above said piston;

valve means in said duct for allowing passage of fluid from the pasesageway to the exterior of the tubing string upon said fluid reaching a predetermined pressure and for preventing passage of fluid through said duct toward said passageway;

a downwardly closing valve assembly operative within said passageway below said duct for preventing fluid from flowing downwardly through said passageway;

releasable coupling means connecting the piston to the lower end of the tubing string; and,

valve means in the piston for closing the vertically extending fluid passageway against movement of fluid in said passageway upward relative to said piston.

9. Apparatus for fracturing a subsurface formation from a well bore in which a casing string is disposed having perforations therein for communication between the interior of the casing string and the formation to be fractured as defined in claim 8 wherein:

the valve means in the piston for closing the vertically extending fluid passageway comprises:

a check valve disposed in the passageway for blocking the upward passage of fluid through the pitssageway;

and further wherein said downwardly closing valve assembly comprises:

an upwardly facing valve seat in the passageway above the check valve; and

a free valve body of a size to pass downwardly through the tubing string and seat on the valve seat.

10. Apparatus for fracturing subsurface formations from a well bore as defined in claim 8 wherein:

the releasable coupling means connecting the piston to the lower end of the tubing string comprises shear- 1 2 able pin means interconnecting the tubing string and the piston. 11. Apparatus for fracturing a subsurface formation from a well bore in which a casing string is disposed having perforations therein for communication between the interior of the casing string and the formation to be frac tured, the apparatus comprising:

from a well bore which comprises:

' piston means slidably received in the well bore;

means for holding said piston means in the well bore above the formation to be fractured;

means associated with said piston means for producing an upward jet of material from the top of said piston means to create a downward thrust on said piston means; and,

means for releasing the piston from the means for holding the piston.

13. The apparatus defined in claim 12 wherein said means associated with said piston for producing an upward jet of material comprises a reaction engine.

14. The apparatus defined in claim 13 wherein said reaction engine includes a bore in said piston having placed therein a quantity of combustible rocket fuel.

15. Apparatus for fracturing'a subsurface formation from a well bore in which a casing string is disposed having perforations therein for communication between the interior of the casing string and the formation to be fractured, the apparatus comprising:

suspension means extending downwardly through the casing;

a piston received within the casing, said piston having an upwardly extended bore and a fluid passageway formed therein;

a quantity of combustible fuel placed within said bore;

means for igniting said fuel associated therewith;

releasable coupling means connecting the piston to said suspension means, said coupling means including a shearible pin;

a check valve disposed in the passageway for blocking the1 upward passage of fluid through the passageway; an

a downwardly closing valve assembly comprising an upwardly facing valve seat in the passageway above the check valve, and a free valve body for cooperating with said seat.

References Cited in the file of this patent UNITED STATES PATENTS 199,488 Alexander Jan. 22, 1878 461,445 Monjeau Oct. 20, 1891 1,103,334 Wilhelmi July 14, 1914 1,248,689 McAvoy Dec. 4, 1917 1,252,557 Dunn Jan. 8, 1918 1,842,107 Lytle Jan. 19, 1932 1,843,002 Small Jan. 26, 1932 2,352,744 Stoddard July 4, 1944 2,806,532 Baker et al Sept. 17, 1957 2,836,249 Freeze May 27, 1958 2,927,638 Hall Mar. 8, 1960 3,066,734 Meiklejohn Dec. 4, 1962 

1. A METHOD FOR FRACTURING A SUBSURFACE FORMATION FROM A WELL BORE COMPRISING THE STEPS OF: PLACING A PISTON IN THE WELL BORE AT A POSITION SUBSTANTIALLY ABOVE THE FORMATION; PLACING IN THE WELL BORE BELOW THE PISTON A GAS AT A FIRST PRESSURE; INCREASING THE PRESSURE OF FLUIDS IN THE WELL BORE ABOVE THE PISTON TO A SUBSTANTIALLY GREATER PRESSURE THAN THE FIRST PRESSURE WHILE SUPPORTING THE PISTON AT SAID POSITION; AND, RELEASING THE PISTON; WHEREBY THE PRESSURE OF THE FLUIDS ABOVE THE PISTON FORCE THE PISTON RAPIDLY DOWNWARDLY TO PRODUCE AN 